1. Field of the Invention
The present invention relates to a system for purifying an exhaust gas. More particularly, the present invention relates to a system for purifying an exhaust gas that improves performance of purifying the exhaust gas by quickly raising an exhaust gas temperature to a temperature at which nitrogen oxide can react with a reducing agent in a selective catalyst reduction apparatus.
2. Description of Related Art
Generally, exhaust gas flowing out through an exhaust manifold from an engine is driven into a catalytic converter mounted at an exhaust pipe and is purified therein. After that, the noise of the exhaust gas is decreased while passing through a muffler and then the exhaust gas is emitted into the air through a tail pipe. The catalytic converter is a type of a diesel particulate filter (DPF) and purifies pollutants contained in the exhaust gas. A catalytic carrier for trapping particulate matter (PM) contained in the exhaust gas is in the catalytic converter, and the exhaust gas flowing out from the engine is purified through a chemical reaction therein.
One type of catalytic converters is a selective catalyst reduction (SCR) apparatus. In the selective catalyst reduction (SCR) apparatus, reducing agents such as urea, ammonia, carbon monoxide, and hydrocarbon (HC) react better with nitrogen oxide than oxygen.
According to a conventional system for purifying an exhaust gas, a catalyzed particulate filter, a diesel oxidation catalyst, and a selective catalyst reduction apparatus are sequentially mounted on an exhaust pipe from an engine. Therefore, the exhaust gas generated in the engine sequentially passes through the catalyzed particulate filter, the diesel oxidation catalyst, and the selective catalyst reduction apparatus, and noxious materials contained in the exhaust gas are removed. That is, the catalyzed particulate filter traps particulate matters (PM) contained in the exhaust gas, the diesel oxidation catalyst oxidizes carbon monoxide and hydrocarbon contained in the exhaust gas into carbon dioxide, and the selective catalyst reduction apparatus reduces nitrogen oxide contained in the exhaust gas into nitrogen gas.
Meanwhile, in order for the nitrogen oxide to react with the reducing agent, an exhaust gas temperature at a front portion of the selective catalyst reduction apparatus is higher than or equal to 200° C. However, since the selective catalyst reduction apparatus is mounted on a rearmost exhaust pipe, long time (e.g., 270 seconds) is required for raising the exhaust gas temperature at the front portion of the selective catalyst reduction apparatus higher than 200° C., as shown in FIG. 5. Therefore, the nitrogen oxide contained in the exhaust gas is hardly reduced and is exhausted to the exterior before the exhaust gas temperature at the front portion of the selective catalyst reduction apparatus is higher than 200° C.
In addition, if the temperature of the exhaust gas generated in the engine is raised by increasing fuel injection amount or by post-injection, exhaust heat is hardly transmitted to the diesel oxidation catalyst and the selective catalyst reduction apparatus at an initial time of raising the exhaust gas temperature because of thermal capacity of the catalyzed particulate filter.
If the fuel injection amount is further increased, excessive hydrocarbon slip occurs in the catalyzed particulate filter.
Therefore, techniques for reducing hydrocarbon slip occurring in the catalyzed particulate filter and quickly raising the exhaust gas temperature at the front portion of the selective catalyst reduction apparatus should be developed.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.